I am making a small robot with an Arduino and Technic LEGO powered by stepper motors. I am planning on moving small rocks throughout my machine. Does anyone have an idea of what kind of Technic LEGO parts I can buy to make a variable length conveyor belt? I have a lot of gears so it's the underlay that's my problem. Does some kind of elastic LEGO parts exist that I can use for this? Chain-like or rubber or hard plastic. IT Would be best if I could make it as long connected as I want.

Maximum size of the small rocks is about the same size as an 1x1 LEGO brick, so the conveyor does not need to have a width of more than 2 cm.

Dropping stones onto your LEGO doesn't sound like a kind way to treat your expensive Technic parts; I imagine that small bits of stone and dirt will fall through a belt of Technic Chain Link or Technic Chain Tread , what with them being a chain rather than a belt. This will then lead to other bits of your contraption becoming polluted with abrasive gunk.

This is the same material that Akiyuki uses as you can see in this low-angle shot up into the tensioning mechanism of the paper conveyor belt in Akiyuki's brick sorter.

Sticky-tape, paper and a smooth LEGO surface underneath the paper to ensure solid support (you don't need to use tiles; try a SNOT technique instead). Presumably you could up the paper to something more robust in the face of having stones dropped on it. You might want to also line the sides of your conveyor to prevent the stones scratching the sides.